1. Field of the Invention
The invention relates to radio systems and, more particularly, to improving the efficiency of frequency hopping as a technique of reducing the effects of fading in an enclosed region of signal propagation.
2. History of the Related Art
Radio Transmission Problems
The quality of the signal received by a mobile station from a base station is affected from time to time by natural phenomena inherent in the use of radio signals to communicate. A factor common to most of the problems related to radio reception is that the desired signal at the receiver is too weak, either in comparison to thermal noise or in comparison to an interfering signal. An interfering signal can be characterized as any undesired signal received on the same channel by the receiver as the desired signal.
In the case of cellular radio systems where all of the frequencies in the available bandwidth are being reused throughout the cellular grid, the efficiency of the radio system is generally limited by the amount of interfering radio signals received rather than thermal noise.
One phenomenon which occurs to limit the quality of a received signal within a radio system is path loss. Even when there are no obstacles between the transmitting antenna and a receiving antenna, the received signal becomes progressively weaker due to the increasing distance between the base station and the mobile station. The received signal power is inversely proportional to a value somewhere between the square and the fourth power of the distance between the transmitting and receiving antennas.
A more common transmission problem in mobile radio systems used in an environment where there are objects such as buildings present, is that of log-normal fading. This phenomenon occurs as a result of the shadowing effect produced by buildings and natural obstacles such as hills located between the transmitting and receiving antennas of a mobile station and a base station. As the mobile station moves around within the environment, the received signal strength increases and decreases as a function of the type of obstacles which are at that moment between the transmitting and receiving antennas. The term "log-normal," comes from the fact that the logarithm of the received signal strength takes the form of a normal distribution about some mean value the minimum values of which are referred to as fading dips and the distance between which may be on the order of 30 to 60 feet.
A third phenomenon which effects signal strength within a mobile system operated in an urban environment is that of Rayleigh fading. This type of signal degradation occurs when the broadcast signal takes more than one path from the transmitting antenna to the receiving antenna so that the receiving antenna of the mobile station receives not just one signal but several. One of these multiple signals may come directly from the receiving antenna but several others are first reflected from buildings and other obstructions before reaching the receiving antenna and, thus, are delayed slightly in phase from one another. The reception of several versions of the same signal shifted in phase from one another results in the vector sum of the signals being the resultant composite signal actually received at the receiving antenna. In some cases the vector sum of the receive signal may be very low, even close to zero, resulting in a fading dip wherein the received signal virtually disappears. In the case of a moving mobile station, the time that elapses between two successive fading dips due to Rayleigh fading depends upon both the frequency of the received signal and the speed at which the mobile is moving. The distance between two fading dips due to Rayleigh fading may be on the order of 7 inches for the 900 megahertz radio band.
Referring to FIGS. 1A and 1B, there is illustrated a perspective model of the frequency/distance received signal fading pattern within a typical mobile radio operating environment. FIG. 1A represents the received signal field of a radio signal operating within an urban area at a frequency of 100 megahertz while FIG. 1B represents a radio signal operating in an urban area with a signal frequency of 300 megahertz. It can be seen from these diagrams how the strength of the signals vary, creating periodic fading dips which are both distance and frequency dependent.
In the case of digital radio systems, such as those in which time division multiple access (TDMA) modulation is used, other radio transmission difficulties arise. One of these difficulties, referred to as time dispersion, occurs when a signal representing certain digital information is interfered with at the receiving antenna by a different, consecutively transmitted symbol due to reflections of the original signal from an object far away from the receiving antenna. It thus becomes difficult for the receiver to decide which actual symbol is being detected at the present moment. Another transmission phenomenon inherent in the use of TDMA modulation is due to the fact that each mobile station must only transmit during a particular allocated time slot of the TDMA frame and remain silent during the other times. Otherwise, the mobile will interfere with calls from other mobiles to which are assigned in different time slots of the same frame.
Conventional Solutions to Radio Transmission Problems
There have evolved a series of techniques which are used to combat the signal degenerative phenomenons which occur in radio transmission systems. One solution which is employed to combat the problems of fading of a digital radio signal, from both log-normal fading and Rayleigh fading, is that of coding and interleaving. This is a technique in which the information representing various items of digital information is organized into blocks, and consecutive ones of a series of blocks, for example, four bits each, are organized into frames. If each of the consecutive bits of information are sent in the same order as they are generated by the speech encoder, the occurrence of a fading dip would totally obliterate several consecutive bits of information which would thus be lost from the communication stream and result in a gap in the speech to be recreated from them. With the technique of interleaving, however, the consecutive bits of information are systematically separated from one another and rearranged in a transmission stream in which they are, rather than contiguous to one another, separated in time from one another with each one forming one bit of a separate block of information. At the other end of the transmission stream the rearranged bits are removed from the blocks of data in which they were transmitted and reconstructed to again be contiguous to one another. When each of these bits representing speech data are separated from the other bits to which they are normally contiguous in time and "interleaved" among other bits not normally contiguous to one another in time and then an entire block of bits is lost from the transmission stream during a fading dip, at least some portion of that lost block can be constructed from the bits which were not lost during the dip because they were interleaved into other blocks which were not lost due to fading. In the case of a moving mobile station, a fading dip only occurs for a very brief period of time as the mobile passes through the region of fading and back into an area of good reception.
One technique used to secure a digital radio transmission against interference is that of error correction coding in which the bits of information to be transmitted are encoded with a correction code so that if bits are lost during transmission they can be recreated with a relatively high degree of accuracy at the receiver site by the error correction code circuitry. A part of the procedure used in correction coding of a digital signal transmission stream is that of interleaving.
An assumption inherent in the use of interleaving techniques with error correction coding is that the mobile is moving so that it passes through a fading dip relatively quickly and only experiences loss of a relatively small block of the digital information due to attenuation while it is located in the region of the fading dip. In the case of transmission environments which are indoors, for example, in a convention center or office building, a mobile station is relatively slow moving or perhaps even stationary. Thus, if the mobile happens to be in a physical location which is subject to a fading dip, it does not pass through that dip quickly and thus a large amount of information is lost due to the fade. Losses of large blocks of information cannot be corrected by mere interleaving and error correction coding.
Another technique used to compensate for transmission difficulties in a radio system is that of frequency hopping. In the use of frequency hopping the radio transmission and reception are at one carrier frequency for one instant of time and then a very short time later the transmission and reception is "hopped" to a different frequency. When a transmitted radio signal is at a different carrier frequency, it is not subject to the same fading pattern because such patterns are frequency dependent and thereby different for different frequencies. Thus, a stationary mobile station which may be in the trough of a fading dip at one carrier frequency might get relatively good reception at a different frequency. In this way, frequency hopping is used to further limit the amount of signal loss to a relatively short segment of the actual transmission time span and thus allow the signal processing circuitry to compensate for the loss of broadcast information with interleaving and error correction coding by reconstructing the lost portions of the transmission.
One important aspect of frequency hopping is that the two or more carrier frequencies between which the signal is successively hopped must each be separated by a certain minimum amount in order to experience independent fading on the different frequencies. In other words, the frequencies between which the signals are hopped must be sufficiently different from one another so that if the received signal is in a fading dip on one frequency it should not be in a fading dip on the other frequency. If the two frequencies are very close together it is more likely that the received signal will be weak due to fading at both frequencies. If, however, the two frequencies are separated from one another by a sufficiently large value then it is less likely that the received signal will be in fading dip on both frequencies. The separation between the two hopping frequencies which is sufficient to obtain independent fading one from the other is called the coherence bandwidth. If each of the two carrier frequencies used for hopping are within the coherence bandwidth, the signals received on each of the two frequencies will be highly correlated. If the carrier frequencies are separated by more than the coherence bandwidth then it is likely that the signals that are received on each of the two frequencies will be uncorrelated and thus, will not be in a fade condition at the same time. If the two frequencies are uncorrelated from one another the radio receiver will probably not experience a fade in the other frequency when one of them is in the fade condition.
When frequency hopping is being used in a TDMA radio signal, and the signal is being hopped across a different frequency during each of the several successive TDMA time slots, with each of the different carrier frequencies being reasonably far away from one another, then it is very likely that the signals received in each of the time slots are completely uncorrelated from one another. Moreover, if error correction coding and interleaving are used across the successive slots in which the signal is received, at least half of the bits received during two successive time slots will not be subjected to a fade condition, in which case the error correction coding and interleaving will do a satisfactory job of reconstructing the complete signal content despite the loss of content from one of the two slots.
In considering coherence bandwidth and correlation factors for frequency hopping applications, the coherence bandwidth is inversely proportional to the time delay spread of the transmission channel. Time delay spread occurs because of multipath propagation. The time difference is between the earliest and latest multipath signals, the main line-of-sight signal and the same signal delayed because one or more reflections creates a time span encompassing both the main signal and its principal echoes which contain most of the signal energy.
The coherence bandwidth of a signal is inversely proportional to the time delay spread of the signal. In the case of indoor implemented radio channels, the time delay spread is extremely small, for example, on the order of 50 to 100 nanoseconds. Thus, the coherence bandwidth for such signals would be very large, for example, on the order of 10 megahertz. Thus, in order to get independent fading in a frequency hopping environment the frequency difference between two carrier signals would have to be on the order of 10 megahertz, far greater than the bandwidth of most practical systems. It is an even greater problem to hop the carrier frequency of the signal over three or four separate frequencies requiring 30 to 40 megahertz of bandwidth.
One prior approach to the coherence bandwidth problem has been to create significant echoes of the broadcast signal by using a second transmitter. By delaying transmission on the second transmitter on the order of a symbol period, the second signal fades independently of the first as a function of the spacing of the second antenna from the first. While the receiver can resolve these two signals a more complex receiver is required in order to deal with the signal echoes. The TDMA signal requires an equalizer while CDMA signals require a Rake combiner.
Another solution has been to use no delay between two separate transmitting antennas, but, rather, to vary the phase differences between them. This causes the fading at the receiver to vary with time. However, due to receiver mobility, the combined effect can cause the fading to change so fast over time that the receiver processing circuity cannot deal with it. For example, if the receiver has a channel tracker for coherent demodulations it may not function properly if the fading changes too quickly over time. Further, if the mobile is designed to be stationary then a time varying phase may not be tracked by the receiver.
There is thus a need for some mechanism to allow the implementation of frequency hopping over a limited bandwidth to correct for flat fading channels in environments such as radio systems implemented in an indoor environment.